The Physics and Psychophysics of Microperimetry

ORIGINAL ARTICLE

The Physics and Psychophysicsof Microperimetry

William Seiple*, Richard B. Rosen†, Veronica Castro-Lima‡, and Patricia M. T. Garcia§

ABSTRACTPurpose. To assess the influences of stimulus parameters (physics) on measures of visual field sensitivity (psychophysics).Methods. Subjects’ thresholds were measured on three different clinically available perimeters: the Humphrey FieldAnalyzer (HFA), the Nidek MP1 (MP1), and the Opko OCT/SLO (OSLO). On all machines, visual field testing was donewith a 10-2 spatial distribution of test points, using Goldmann Size III and Size I stimuli, with a presentation time of 200ms, and using a 4-2 threshold algorithm.Results. All the MP1 and OSLO data fell below the values for the corresponding points on the HFA. For the GoldmannSize III target, the HFA median threshold was 33 dB, whereas the MP1 median threshold was 19 dB and the OLSO, 18dB. Using the increment intensity values at each dB level for each microperimeter, the data were converted to equivalentHFA dB. Using this conversion, the smallest increment displayed in the MP1 (1.27 cd/m2) was equivalent to 34 HFA dB,and the brightest increment displayed by the MP1 was 14 HFA dB (127 cd/m2). The smallest increment displayed in theOSLO (1.56 cd/m2) was equivalent to 33.1 HFA dB, and the brightest increment displayed by the OSLO was 13.6 HFAdB (137 cd/m2). There was good correspondence among these results when compared using equivalent incrementthreshold units. However, discrepancies in our findings made us acutely aware of the importance of evaluating theconsequences of design choices made by the manufacturers.Conclusions. The findings underscore the need for users to check their assumptions about what the equipment is doingand to always evaluate the psychophysical consequences of the stimuli that are used by a particular instrument.(Optom Vis Sci 2012;89:1182–1191)

Key Words: microperimetry, low vision

Perimetry has long been a part of clinical vision examina-tions,1 and in the past decades, static automated perimetry,such as the Zeiss Humphrey Field Analyzer (HFA), has

become a “gold standard.”2–8 Threshold visual fields have beenextremely useful for diagnosis and prognosis in patients with dis-eases of the visual system.9 However, in patients with eccentricviewing and/or unsteady fixation, the uncertainty about the exactposition of the retina during testing reduces the clinical utility ofthese perimetry tests.10–21 In an attempt to address these problems,Scanning Laser Ophthalmoscopes (SLO) have been modified toallow fundus viewing while conducting perimetric tests. These

instruments have not been widely available and/or easily modifiedfor clinical work.15,18,22–26 Newer, clinically friendly perimetryinstruments, such as the Nidek Inc. MP1 (MP1) and Opko Instru-mentation Spectral OCT/SLO (OSLO) microperimeter track fixa-tion and correct target placement for eye movements during testing.These instruments allow for precise microperimetric assessment offield sensitivities (i.e., fundus-guided perimeter).7,26–28

There are many similarities between the new microperim-eters and standard clinical perimeters (such as the HFA), in-cluding similar spatial patterns, similar threshold algorithms,and similar stimulus sizes and durations. However, there areimportant differences in stimulus configuration that may pro-duce discordant results. For example, the HFA perimeter uses aprojection system with a broad range of stimulus intensities,whereas the MP1 and OSLO microperimetry instruments usesmall solid-state monitors to present the targets over a limitedrange of intensities. Stimuli are presented on a background of10 candela per meter square (cd/m2) in the HFA and OSLO,but on a background of 1.27 cd/m2 in the MP1.

*PhD†MD‡MD§MDLighthouse International, New York, New York (WS), New York Eye and Ear

Infirmary, New York, New York (WS, RBR, VCL, PMTG), Jesse Brown VAMC,Chicago, Illinois (WS), Department of Ophthalmology, New York UniversitySchool of Medicine, New York, New York (WS), and New York Medical College,Valhalla, New York (RBR).

1040-5488/12/8908-1182/0 VOL. 89, NO. 8, PP. 1182–1191OPTOMETRY AND VISION SCIENCECopyright © 2012 American Academy of Optometry

Optometry and Vision Science, Vol. 89, No. 8, August 2012

Given the increasing availability of microperimetry instruments, weillustrate the consequences of the choices of stimuli and the influencesof hardware choices made by each particular equipment on perimetryresults. To do this, we compare the results obtained with two micro-perimetry instruments (MP1 and OSLO) to those obtained with anHFA. The purpose of this study was not to collect normal values on alarge number of subjects or to demonstrate disease influences onthresholds; many studies of this type have been previously pub-lished.26,29,30 The data of this study were used to give a reader anunderstanding of the relationships among the measures obtained fromeach instrument, to allow direct comparison of results on a standardscale, and to discuss the psychophysical implications.

METHODS

Subjects

We recruited six normally sighted subjects to participate in thisstudy. Inclusion criteria were 20/20 or better acuity, refractiveerrors not exceeding �5.00 D sphere and �2.00 D cylinder, no

history of ophthalmologic or neurologic disease, and undilatedpupil diameters of at least 4 mm. Subjects gave informed consentto participate, and the research was approved by the NYEE IRB.The study followed the tenets of the Declaration of Helsinki. Fivenormally sighted subjects with a mean age of 25.8 years partici-pated in the study. The right eye of each subject was tested.

Procedures

Subjects’ thresholds were measured on the HFA, Nidek MP1(MP1), and OSLO (Fig. 1). Pupil were not dilated for any of thetests and pupil diameters were measured for each subjects duringtesting. The order of testing was counterbalanced across subjects.On all machines, visual field testing was done with a 10-2 spatialdistribution of test points, using Goldmann Size III and Size Istimuli, with a presentation time of 200 ms, and using a 4-2 thresh-old algorithm. These two test sizes were chosen to allow examina-tion of the sensitivities over the entire range of physical stimuliavailable in the microperimetry instruments. Two tests per subject

FIGURE 1.Examples of the raw data obtained from the three perimeters used in this study. (A) Zeiss Humphrey Field Analyzer (HFA). (B) Nidek Inc. MP1 (MP1).(C) Opko Instrumentation Spectral OCT/SLO (OSLO).

The Physics and Psychophysics of Microperimetry—Seiple et al. 1183


at each size were performed on each machine. Subjects were givenrest periods after each test. The order of testing was counterbal-anced. Testing was done in two sessions of approximately 2 h each,conducted on 2 consecutive days.

The individual parameters were as follows:

Humphrey Field Analyzer

The background level of the HFA was 10 cd/m2 (31.5 apostilbs(asb)).31 The average pupil diameter for our subjects under a10-cd/m2 background was 5 mm. HFA stimuli can be presented overa range intensities from a maximum of 10,000 asb (3183 cd/m2) to a5 log-unit attenuated value of 0.01 asb (0.03 cd/m2).31 The standardviewing distance of 30 cm was used. Fig. 1A shows the output of atypical HFA field test on a normally sighted subject. This output plotsthresholds in dB attenuation as a function of visual field location.There were 68 test points presented in the HFA perimetry.

Nidek MP1

The background of the MP1 was nominally 1.27 cd/m2 (4 asb).29

The average pupil diameter for our subjects under a 1.0-cd/m2 back-ground was 6 mm, which equates to an adapting illuminance of 1.52log td in the MP1. The maximum stimulus intensity was 127cd/m2 (400 asb), attenuated over a 2-log-unit range.29 The MP1has an automated tracking system that shifts the position of thestimulus to compensate for movement of the eye. An image of theretina was taken, and a landmark region of 128 � 128 pixels wasselected for tracking. During testing, infrared images were alignedto the reference image and stimulus positions adjusted accordingly.The time for tracking was �2 frames (80 ms).29 Fig. 1B shows thethreshold values for a MP1 test overlaid on an image of the fundus.There were 68 test points presented in the MP1 perimetry.

Opko Instrumentation Spectral OCT/SLO

The background level of the our OSLO was 10 cd/m2 (31.5 asbto 2.29 log td), with a maximum stimulus intensity of 139.8 cd/m2

(439 asb), attenuated over a 2-log-unit range. The OSLO instru-ment employs a scanning laser to image the fundus and tracks eyemovements by aligning a subset of fundus landmarks with a refer-ence image. The images of the fundus and organic light-emittingdiode are coherent; thus, eliminating concerns of misalignment.32

Stimulus position was adjusted, and presentation made in �100 ms.Fig. 1C shows the threshold results plotted on an OSLO image of thefundus. There were 52 test points presented using the OSLP perimeter.

RESULTS

Comparisons

Both the HFA and MP1 present 68 stimuli at equivalent fieldlocations. For analyses between these instruments, all of the datawere included. The OSLO presents only a 52 point subset of thepoints displayed on the other instruments. For all analyses withOSOL data, only the spatially equivalent 52 data points from theHFA and MP1 were used.

MP1 vs. HFA

In Fig. 2, the data for MP1 are plotted against the HFA data. Foreach subject, the averages of the two runs on each instrument foreach stimulus position are plotted. The MP1 data were transposedinto field view, and spatially corresponding data points were com-pared for the two tests. All the MP1 data fell below the values fortheir corresponding points on the HFA.

For the Goldmann Size III target (Fig. 2A), HFA thresholdsranged from 25 to 37 dB (median � 33 dB) across the field,

FIGURE 2.Plot of the HFA and MP1 thresholds (in dB attenuation) for the Goldmann Size III (A) and Goldmann Size I (B) targets.

1184 The Physics and Psychophysics of Microperimetry—Seiple et al.


whereas MP1 thresholds ranged from 12 to 20 dB (median � 19dB). The Spearman correlation between these two measures was0.24. For the MP1 data, there was a floor effect (a luminanceincrement lower than that of the 20 dB nominal intensity couldnot be presented in this instrument), with a disproportionate num-ber of points falling at an MP1 value of 20 dB (from 35 to 61% ofall points across subjects). The values of corresponding HFA pointsranged from 27 to 37 dB.

For the Goldmann Size I target (Fig. 2B), HFA thresholdsranged from 14 to 30 dB (median � 24 dB), whereas MP1 thresh-olds ranged from 2 to 16 dB (median � 8.5 dB). All the MP1 datafell below the values for their spatially corresponding points on theHFA. The Spearman correlation between these two measures was0.56.

When averaged into eccentricity rings, median thresholds de-creased as a function of eccentricity for both HFA and MP1 (Fig.3). There was no statistical difference in slope between the HFA orMP1 dB and ring eccentricity for either target size.

OSLO vs. HFA

In Fig. 4, the data for OSLO are plotted against the HFA data.The OSLO data were transposed into field view, and spatiallycorresponding data points were compared for the two tests. Foreach subject, the averages of the two runs on each instrument foreach stimulus position are plotted. Many data points overlap dueto the 4 to 2 threshold algorithm used.

For the Goldmann Size III target (Fig. 4A), HFA thresholdsranged from 25 to 37 db (median � 33 dB) across the field,whereas OSLO thresholds ranged from 12 to 20 dB (median � 18dB). All the OSLO data fell below the values for their spatiallycorresponding points on the HFA. The Spearman correlation be-tween these two measures was 0.49. OSLO thresholds decreased asa function of eccentricity at a rate that was not statistically differentthan the HFA data (Fig. 3; t � 0.91, p � ns).

For the Goldmann Size I target (Fig. 4B), HFA thresholdsranged from 14 to 30 dB (median � 24 dB), whereas OSLO

0

10

20

30

40

0 1 2 3 4 5

dB

Eccentricity (Rings)

A

HFA

MP1

OSLO

0

10

20

30

40

0 1 2 3 4 5

dB

Eccentricity (Rings)

B

HFA

MP1

OSLO

FIGURE 3.Plot of the median threshold for each perimeter as a function of eccentricity for the Goldmann Size III (A) and Goldmann Size I (B) targets. There wasno significant differences among the slopes of threshold change with eccentricity for any of the instrument.



thresholds ranged from 6 to 18 dB (median � 14 dB). All but four ofthe OSLO data fell below the values for their spatially correspondingpoints on the HFA. The Spearman correlation between these twomeasures was 0.38. For the Goldmann Size I target, OSLO thresholdsdecreased as a function of eccentricity at a rate that was not statisticallydifferent than the HFA data (t � 0.57, p � ns).

MP1 vs. OSLO

In Fig. 5, the data for MP1 are plotted against the OSLO data.For each subject, the averages of the two runs on each instrumentfor each stimulus position are plotted. For the Goldmann Size IIItarget, the MP1 thresholds ranged from 12 to 20 dB (median � 19

FIGURE 4.Plot of the HFA and OSLO thresholds (in dB attenuation) for the Goldmann Size III (A) and Goldmann Size I (B) targets.

FIGURE 5.Plot of the OSLO and MP1 thresholds (in dB attenuation) for the Goldmann Size III (A) and Goldmann Size I (B) targets.



dB), and the OSLO thresholds also ranged from 12 to 20 dB(median � 18 dB) (Fig. 5A). The Spearman correlation betweenthese two measures was 0.28. For the Goldmann Size I target (Fig.5B), the MP1 thresholds ranged from 2 to 16 dB (median � 8.5dB), and the OSLO thresholds ranged from 6 to 18 dB (median �14 dB). The Spearman correlation between the two measures was0.54.

Patient Comparison

All of the OSLO data from the current study (Size I and IIIcombined) were regressed against the HFA data. The slope of thisfit was 0.327 and the intercept was 5.0 db. HFA and OSLO datafrom 20 eyes with glaucoma were available from the report of Lima

et al.33 The range of HFA data for these patients was from 30 to 6dB and the range for the OSLO was 18 to 6 dB. This range of dBvalues was very similar to the range of values across target sizescollected in the present study. A regression fitted to the patient dataof Lima et al. yielded a slope of 0.327 and an intercept of 4.5 db.There were no statistically significant difference between the slopesor intercepts for the control and patient data.34 This finding pointsto the generalizability of the findings of current study to studiesreporting data from patients.

Comments

The MP1 and OSLO threshold values were consistently lowerthan the HFA values at corresponding retinal locations, and there

TABLE 1.Test increments (Delta I)

Humphrey(db)

Delta I(cd/m2)

MP1(db)

Delta I(cd/m2)

Equivalent(HFA dB)

OSLO(db)

Delta I(cd/m2)

Equivalent(HFA dB)

40 0.339 0.438 0.537 0.636 0.835 1.034 1.3 20 1.3 34.0 20 1.6 33.133 1.6 19 1.6 33.0 19 2.1 31.832 2.0 18 2.0 32.0 18 2.6 30.931 2.5 17 2.5 31.0 17 3.1 30.130 3.2 16 3.2 30.0 16 4.0 29.029 4.0 15 4.0 29.0 15 5.1 28.028 5.0 14 5.1 28.0 14 6.4 27.027 6.4 13 6.4 27.0 13 8.0 26.026 8.0 12 8.0 26.0 12 10.1 25.025 10.1 11 10.1 25.0 11 12.7 24.024 12.7 10 12.7 24.0 10 16.0 23.023 16.0 9 16.0 23.0 9 20.1 22.022 20.1 8 20.1 22.0 8 25.5 21.021 25.3 7 25.4 21.0 7 31.9 20.020 31.8 6 31.9 20.0 6 40.1 19.019 40.1 5 40.2 19.0 5 50.7 18.018 50.4 4 50.6 18.0 4 63.6 17.017 63.5 3 63.8 17.0 3 80.2 16.016 80.0 2 80.3 16.0 2 100.8 15.015 100.7 1 101.0 15.0 1 127.2 14.014 126.7 0 127.0 14.0 0 135.0 13.713 159.512 200.811 252.810 318.39 400.78 504.57 635.16 799.55 1,006.64 1,267.23 1,595.32 2,008.31 2,528.30 3,183.0



was not a one-to-one correspondence among these data. Thismight reflect the differences in scales, with the microperimeters’attenuation values ranging from only 0 to 20 dB. But, it is moresubtle than this. The dB values for each test represent attenuationfrom different maximum values. In the HFA, attenuation is calcu-lated from a maximum stimulus intensity of 10,000 asb, whereasthe microperimeters maximum values are approximately 2 log unitdimmer. In order to accurately compare these tests, all the datashould be converted into equivalent threshold values. Because theHFA data are a “gold standard” for perimetry, we converted theMP1 and the OSLO data into equivalent HFA values. In psycho-physics, thresholds are standardly expressed as increments, mea-sured in delta intensity units above the background level. Table 1lists the published HFA,31 MP1,29 and measured OSLO incre-ment intensities at each instrument’s dB level. The conversion toHFA equivalent values was accomplished using the formula:

HFA equivalent � �(log(�/3183) � 10)

where 3183 is the maximum intensity of the HFA perimeter incd/m2; � is the value of the microperimeter (MP1 or OSLO)increment in cd/m2; and the mulitplier 10 converts the valueto dB.

Using this conversion, the smallest increment displayed in theMP1 (1.27 cd/m2) was equivalent to 34 HFA dB, and the brightestincrement displayed by the MP1 was 14 HFA dB (127 cd/m2).The smallest increment displayed in the OSLO (1.56 cd/m2) wasequivalent to 33.1 HFA dB, and the brightest increment displayedby the OSLO was 13.6 HFA dB (137 cd/m2) (Fig. 6).

MP1 Equivalent vs. HFA

In Fig. 7, the data for MP1 thresholds, converted to equivalentHFA dB based upon their increment intensity values, are plottedagainst their spatially corresponding HFA-measured values. Forthe Goldmann Size III target (Fig. 7A), MP1 thresholds rangedfrom 26 to 34 equivalent HFA dB (median � 33 dB). For theGoldmann Size I target (Fig. 7B), MP1 thresholds ranged from 14to 28 equivalent HFA dB (median � 21 dB). For both Goldmannsizes, the data clustered around the line of unity.

OSLO Equivalent dB vs. HFA

In Fig. 8, the data for OSLO thresholds converted to equivalentHFA dB are plotted against their corresponding HFA-measureddata. For the Goldmann Size III target (Fig. 8A), OSLO thresholdranged from 25 to 34 equivalent HFA dB (median � 30 dB). Forthe Goldmann Size I target (Fig. 8B), OSLO threshold rangedfrom 19 to 31 equivalent HFA dB (median � 27 dB). For thelarger target size, the data clustered around the line of unity,whereas the OSLO data for the smaller target were generally above(lower thresholds) than the HFA data.

Repeatability

To calculate the agreement within these perimeters, the differ-ences between repeated tests on the same instrument for the Size IIItarget were plotted against the average values as suggested by Blandand Altman.35 The agreement between repeated tests on the sameinstrument was calculated as the standard deviations of the differ-ences. For the Size III target, the agreement was 1.6 dB for repeatedmeasurements on the HFA, 2.0 dB for the MP1, and 2.5 dB for theOSLO. For the Size I target, the agreement between two repeatedmeasurement was 2.6 dB for both the HFA and MP1 and 2.3 dBthe OSLO.

As a comparison, agreement in thresholds between instrumentsfor the Size III target was similar in magnitude to that within aninstrument. Agreement between the HFA and MP1 was 2.8 dB,between the HFA and OSLO it was 2.3 dB, and between the MP1and OSLO it was 2.4 dB. For the Size I target, agreement withininstruments was better than between instruments. Agreementbetween the HFA and MP1 was 3.3 dB, between the HFA andOSLO it was 2.8 dB, and between the HFA and OSLO it was3.6 dB.

DISCUSSION

The manuscript examined the relationship among thresholdsobtained on three visual field instruments for the same sample ofcontrol individuals. We attempted to cover much of the availableranges of physical stimulus intensities available in these instru-ments by changing stimulus target size, rather than by includingpatients with diseases that could affect sensitivities. We used asmaller target size to extend the range of stimulus intensities.Across stimulus size, our range of threshold values for the HFA wasfrom 37 to 14 dB, for the MP1, thresholds ranged from 20 to 2 dB,and for the OSLO, thresholds ranged from 20 to 6 dB. Thesevalues covered most of the ranges of intensities for the two micro-perimeters and much of the HFA range.

FIGURE 6.Plot of increment intensities in cd/m2 for the OSLO (dashed line) and HFA(solid line) against their corresponding dB values. Arrows show equiva-lence of OSLO 20 dB increment value to an increment value of 33.1 dBin the HFA.



Our goal was to discuss the relationships among the testingalgorithms and stimulus parameters, rather than to examine therelationship between sensitivities and disease. We found that, otherthan the floor effect, there was a linear relationship between instru-ments from very small to very large luminance increments. Thepresent data would be invalid only if a disease caused differences inthresholds to stimuli of the same physical intensities when pre-

sented in different instruments. We have presented evidence fromone group of patients that the relationships between instrumentsholds for patients as well as for targets of different sizes.

We found that a comparison among the results obtained withthe three instruments did not yield unitary correspondence,mainly due to the differences in stimulus parameters. However,when all the data were expressed as equivalent HFA dB, there was

FIGURE 7.Plot of the HFA (dB) and MP1 thresholds (in equivalent HFA dB) for the Goldmann Size III (A) and Goldmann Size I (B) targets.

FIGURE 8.Plot of the HFA (dB) and OSLO thresholds (equivalent HFA dB) for the Goldmann Size III (A) and Goldmann Size I (B) targets.



a one-to-one relationship among most of the measures. One ex-ception to this trend was the Goldmann Size III data from theMP1. There was a floor effect observed in these data, with many ofthe MP1 data points falling at the lowest increment value (20 dB).This was not observed in the OSLO data. This was most likelycaused by the dimmer background used in the MP1 relative to theother two instruments. A 20-dB stimulus in the MP1 was equiva-lent to a 1.27 cd/m2 increment above the background, whereas a20-dB OSLO stimulus was equal to an increment of 1.56 dB. Atfirst glance, it would appear that the MP1 should be more sensitiveat lower threshold levels due to its smaller increment. However,when the background illuminance is taken into account, theWeber contrast of the MP1 20-dB stimulus was 1.0 (Delta I /Back-ground I), whereas the Weber contrast of the OSLO 20-dB stim-ulus was 0.15. As a comparison, the calculated median centralHFA threshold was 35 dB for a Goldmann Size III target, whichwas equivalent to a Weber increment of 0.1. This means that thefirst contrast step displayed in the MP1 was considerably largerthan the minimum threshold increment needed for a normallysighted subject to see the target. In fact, a Weber contrast of 1.0 wasequivalent to a threshold of approximately 24 HFA dB. This mayexplain the correspondence of MP1 20-dB thresholds with HFAvalues between 25 and 35 dB.

Background illuminance plays a large role in determining incre-ment thresholds. Reports on threshold vs. intensity functions havebeen published for human psychophysics and electrophysiology.36–38

In these functions, there is a range of lower adapting illuminancelevels over which thresholds are unchanged. Luminance gain (lin-earity of the Weber function) begins at around a background illu-minance level �2.0 log td.37 The low background level used in theMP1 was approximately 0.5 log unit below the level required forlinearity and well into the mesopic range assuming an undilatedpupil diameter of 6 mm. When measuring thresholds at adaptingilluminance levels in the mesopic range, thresholds can be medi-ated by mixed rod-cone system responses and by the interactionsbetween photoreceptor types.39–43 As a consequence, thresholdsmeasured under mesopic conditions will vary with stimulus spec-tral content, retinal location, spatial frequency, and temporal prop-erties.43

Although most of the data aligned with the HFA results whenconverted to equivalent HFA dB, the data for the Goldmann SizeI target in the OSLO fell consistently above the line of unity. Thisindicates that threshold increments were lower for this size target inthe OSLO instrument than in the HFA. Ricco’s Law states thatthreshold is determined by the product of stimulus area and lumi-nance.44 Within a critical area, the product is constant. In ourstudy, median thresholds increased between the Goldmann Size IIIand Goldmann Size I targets for the HFA from 33 dB (1.6 cd/m2)to 23 dB (12.64 cd/m2). The area of a Goldmann Size I stimulus issixteen times smaller than the area of the Goldmann Size III stim-ulus. The median increment energy needed to reach thresholds forGoldmann Size I targets was 9.8 times higher than that required forthe Goldmann Size III in the HFA, indicating incomplete spatialsummation. This deviation from Ricco’s Law is most likely due tothe use of relatively large stimulus sizes and the fact that we aver-aged over a range of eccentric locations.45,46 For the MP1, thresh-olds increased from a median of 19 dB for the Goldmann Size IIItarget to 8 dB for the Goldmann Size I target, or an increase in

luminance of 10.1 times, again demonstrating less than completesummation. For the OSLO data, the change in median thresholdsbetween the two stimulus sizes was only 3 dB (from 17 to 13 dB),or an increase in threshold energy of only 2.6 times. One explana-tion for the discrepant finding with the OSLO might be that thesize of the Goldmann Size I stimulus in the OSLO was too large.After consulting with the manufacturer, it was discovered that theGoldmann Size I target presented by OLSO was 2.5 times larger inarea than a standard Goldmann Size I target. This error has beencorrected in newer versions of their software.

SUMMARY

The findings of our work emphasize the importance of compar-ing data using equivalent scales. There was good correspondenceamong these results when compared on equivalent incrementthreshold units. However, discrepancies in our findings allowed usto evaluate consequences of design choices made by the manufac-turers. The lesson to be learned from this work is that users shouldmake no assumptions about what the equipment is doing andshould always evaluate the psychophysical consequences of thestimuli that are used by a particular instrument.

ACKNOWLEDGMENTS

This work was supported in part by a grant from the U.S. Department ofVeterans Affairs and the Bendheim Family Retina Center of the New York Eyeand Ear Infirmary.

Received December 28, 2011; accepted April 25, 2012.

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The Physics and Psychophysics of Microperimetry